My solution is for simply checking whether you get a valid date object:

Implementation

Date.prototype.isValid = function () {
    // An invalid date object returns NaN for getTime() and NaN is the only
    // object not strictly equal to itself.
    return this.getTime() === this.getTime();
};  

Usage

var d = new Date("lol");

console.log(d.isValid()); // false

d = new Date("2012/09/11");

console.log(d.isValid()); // true